Genetic Similarity, Inbreeding and Hatching Failure in Blue Tits: Are Unhatched Eggs Infertile?

We use data from a long-term population study in combination with DNA fingerprint data to study the frequency of inbreeding and its effects on reproductive parameters in a blue tit population. Close inbreeding was very rare in this population. The proportion of unhatched eggs in a clutch was related to the degree of genetic similarity between the parents as determined by multilocus DNA fingerprinting. Data from blue and great tit populations studied over 15 years show that about 25—30% of blue tit and 20 % of great tit nests contained at least one unhatched egg. The number or proportion of unhatched eggs in the nest was highly repeatable for pairs breeding in different years, but not for individual males or females. Unhatched eggs, therefore, were unlikely to result from functional infertility. The hypothesis that female blue tits engage in extra-pair copulations as insurance against their mate’s infertility can thus be discarded. Because the genetic similarity between the female and the extra-pair male was not lower than that between the female and her social partner, our data do not support the hypothesis that females engage in extra-pair copulations to reduce inbreeding depression.